1. Field of the Invention
This invention relates to color cathode ray picture tubes, and is addressed specifically to an improved system for mounting the support frame for a tension mask in a flat tension mask (FTM) cathode ray tube (CRT).
A tension foil shadow mask is a part of the CRT front assembly, and is located opposite the viewing side of the faceplate in close adjacency to the faceplate. A shadow mask for an FTM CRT comprises an apertured metallic foil which may, by way of example, be about 0.001 inch thick, or less. As is well known in the art, a shadow mask acts as a color-selection electrode, or "parallax barrier," that ensures that each of the three beams generated by the electron gun located in the neck of the tube lands only on assigned phosphor targets.
The mask is welded to and supported in high tension at a predetermined distance from the inner surface of the face panel by a rectangular mask frame that extends from and is secured to the faceplate. The purpose of the frame, which encloses the centrally located phosphor screen, is to support the mask a predetermined, exact distance from the screen, a dimension known as the Q-distance. A mask frame may comprise a one-piece unitary structure, or it may be made up of four discrete support segments in facing relationship and joined at the corners. Alternatively, the frame may consist of a plurality of unjoined, discrete segments as The sides of the frame are referred to individually as "rails." The apertured foil that comprises the mask is stretched over the rails and welded to the top of a mask-receiving surface on the mask frame.
The mask frame must be of high strength to withstand the high tension of the mask after it is stretched over the frame and welded to the frame. A loose frame or an inward tilt of the frame under the tension forces of the mask can cause misregistration of the mask apertures with the phosphor deposits on the screen, resulting in color impurities in the picture display. Also, the mask frame must be affixed securely to the glass of the faceplate to derive strength therefrom, thus allowing for a smaller and lighter frame which need not independently support the tensioned shadow mask.
A mask frame can be secured to a faceplate by a cement such as a devitrifying solder glass, as disclosed in U.S. Pat. No. 4,695,761 to Fendley, commonly owned herewith. In its devitrified form, the solder glass is in a crystalline state which is irreversible, and it will remain a solid adhesive during ensuing, high-temperature production operations. While effective as a means of attachment of a mask frame to the glass of a faceplate, the solder glass and the frame are subject to heat expansion and shrinkage during the high-temperature devitrifying process which affects its flatness and the resulting accuracy of Q-height. Also, solder glass has a high flow rate during heating and forms a bead of glass which reduces the usable screen area. Further, it is difficult to handle and apply, and the devitrifying process entails an additional production step with concomitant high temperature.
FTM frames are usually made of a metal alloy that is compatible with the inner environment of the cathode ray tube The alloy is selected to have a coefficient of thermal expansion (CTE) compatible with that of the glass of the faceplate. An example of an all-metal frame is disclosed the afore-cited '761 patent. Alternatively, the mask frame may be made up of a ceramic base which is secured to the faceplate, and topped with metal cap to which the mask is attached by means such as welding. A mask frame of this type is described and claimed in U.S. Pat. No. 4,891,546, also commonly owned.
As is known, the screening of faceplates is currently accomplished by the mating of a shadow mask and faceplate, and photoexposing a series of chemically sensitized coatings on the screening area that is located on the inner surface of the faceplate. The resulting screen consists of a black deposit that surrounds discrete phosphor deposits. This process is described in U.S. Pat. No. 3,973,964 to Lange, of common ownership. While this manufacturing process can produce excellent screens, it is expensive and time-consuming because of the number of steps required, and the need for elaborate production machinery operated by highly skilled personnel.
Direct contact printing of screen elements to CRT faceplates, in which the matrix and the phosphor deposits are accurately formed on the screening area by some form of printing, is a viable alternative for an interchangeable mask system; that is, a system in which masks and screens do not have to be mated, and in which any mask can be used with any screen. A process of this type is set forth in referent copending application Ser. No. 07/654,843 entitled "Method and Apparatus for Direct Contact Printing Screens on CRT Faceplates."
In the direct contact printing of flat tension mask tubes, the process of printing on the faceplate can be hindered by the presence of the mask frame, which heretofore had to be permanently installed on the inner surface of the faceplate prior to the printing. An installed frame acts as a barrier to the printing roller or other screen printing means as it must be designed so it will not be impeded by the mask frame and the bead of solder glass that surrounds it, an expedient which unduly complicates the printing process. Ideally, the screen is printed on the screening surface of a flat faceplate before the mask frame is installed.
An "open" frame; that is, one that is not unitary, but has openings at the corners, for example, complicates the application of screening slurries by the spin-coating process. In this process, the screen is rotated while a grille or phosphor slurry is poured on the center of the screen. A uniform application of the phosphor results from the spinning. The presence of an open frame mask is an obvious impediment because phosphor particles are entrapped in the frame, and the coatings resulting from the spin-off of the slurries is uneven.
2. Related Art
In U.S. Pat. No. 4,716,334 to Fendley, there is disclosed a mask frame in which the metal of the frame is caused to physically penetrate the glass of the faceplate, obviating the need for attachment by a cement such as solder glass. A frame such as this impedes the printing of a screen because it is an integral and unremovable part of the faceplate prior to the screening.
Lopata et al in U.S. Pat. No. 4,900,977, discloses a reference and support system for a flat CRT tension mask. A mask frame is shrink-fitted onto a peripheral surface of the face panel surrounding the target area, or screen. The upper edge of the frame is finished so as to provide the desired Q-spacing between the screen area and the top of the support frame where the mask is to be attached. A temporary fixture is described which is used in applying various phosphors to a target area, and in positioning a tensed shadow mask on the support frame. The temporary fixture mechanically registers with the support frame. An embodiment is also disclosed in which the base of the mask frame is shown as being emdedded in the faceplate at its periphery.
The embedment of metal in glass is the subject of U.S. Pat. Nos. 2,949,702 to Blanding et al and 3,417,274 to Bennett et al.
3. Other Related Art
U.S. Pat. No. 4,725,756 to Kaplan, of common ownership herewith.